Differential relay



Dec. 14, 1948; E TO K 2,456,541

DIFFERENTIAL RELAY Filed Oct. 24, 1945 will in part be obvious, and in part appear here- -inafter. 1

Patented Dec. 14, 1948 "YSTATES I DIFFERENTJALBEL Y Josef -Stoe'cklin, Baden, Switzerland; assignor to @Aktiengesellschaft BroWn,-Bo'veri'& C-ic; Baden, .-.1Switzerland, a joint-stock icompan'y of'iSwitzcr- :Application October 24,1945 Serial No.11624fl89 In SwitzerlandiObtQberZS; 1944 Claims. (01138155294) vThiS *inventionrelates;generally to differential ela'y;protectivesystems fortransformers and the ike andit has particular relation to apparatus for preventing the false: operation-thereof on. ini- 1 tial energization of .the transformer.

' ,.',V.arious; arrangements have been provided to prevent-the false .operation of protective relays .for transformers when the transformer is con- -nec-ted intothecircuit; The primary purpose of VI these-relays is to-protectthe transformer from the .damage incident to an internal fault but certain of theoperating'conditionspwhich exist on athe occurrence of an internal faultare also present when the transformer is connected to the circuit for energization.

One manner of-preventingfalse actuation on .switching in is to lessen the sensitivity of the .....differential relay :system. This can be accom- 1 plished by means of auxiliary contacts on the cir ,,.cuit breakers on switches.

, It isalsowellknown to "employ certain charac- .,-'.teristics ofthe direct current components in the magnetizing current or the higher harmonics in proposed for, accomplishing this purpose but, in

I .general,,they. have ,beenunsatsifactory for various reasons. Theyhavebeen complicated, have I interfered withthe functioning of the, differential protectivesystem on the occurrence of an internalfault, and have,interferedwiththe proper functioning of the system when it should infact operate.

An object of this invention is to prevent im- 1 --propergfunctioning of a differential relay protective system fora transformer or like inductive apparatus.

Another object is to close the tripping circuit for the switches or circuit breakers connecting the transformer in the circuit, only on theoccur- :rence ofan internal fault therein.

- then flowing would otherwise effect operation of the differential relay protective means.

A further object is to employ a current frequency foreign to the normal current frequency for effecting the neutralization of the differential relay protective means.

--Other objects and advantages of this invention i di 2 z-- "Ihe invention tistdisclosed imthe'. embodiment thereof1 shown in .the. acompanying drawing. and 1 itcompriseszthe. features of ;.construction, combination. oieelements tand arrangement, of. parts whichcwill, berexemplified inathei 'constructions t 1. hereinafter..set.fortheandltherscope of the application ofwhich will ;be indicatedinthe appended eclaims.

. .Fonaimore completerunderstanding of the nature..and..scopeyoflthis invention, reference can 2,. .be had-tothe following description taken together 2 with .the ;accompanying:;drawings, in which: .zFig.(liillustrates, diagrammatically, one embodiment ofthe present invention as incorporated i5.ineaxconventional,dififerential relay system for a a. transformer; :l: Figs. 2,- and :3 areqitime-current. curves which how ,graphicallycertain operating characteristics ofaethe system. .:Referring now particularly to vFig. 1, the referencecharaoter l0- designates an alternating cur- -,,rent, generator, .such as a'60- cyc1e generator, {HQWhiCh isconnected by a-switch or-circuit-breaker 1', H, to:energize the powertransformer l2,,having ,a primary winding and a secondary winding 4' .-;which is-connected by a switch, or circuit 1 breaker; I5,- to energize a load circuit as shown. 0 Trip coils I!v and I!) are provided for the circuit breakers llland li, respectively, to effect opera- ?Qation thereof to the open circuit position.

For convenience, theoperating means for closing thefcircuit breakers H and {5 has not been hown since it forms no part of the present inentionr While 8. tsingle phase system has been -i llustrated,- it..wi1l be.understood..that the invenion is applicable to a-polyphase systemv as well. accordance, "with the conventional practice,

urrenttransformers;shown generally at 2 0 and g 2| are connected in opposed relation on opposite f g-sides of the transformer |2.- 'When'the current n -';fi0W- togand-;from---the-;transformer isthe same, I i ofecourse; taking 'into. consideration :the ratio of ransformation,ethen'the voltage at the :itermi- 1.?nals of.-:.the:current transformer: 21! is equal; and oppositeto-the 1voltage air-lithe terminals of :the ycurrenttransformer 2 I. -The current transformers 20-and Zl-are connected tothewindin'gs 24 and 25 0f adifferential relay 26 having normally 5 open contacts Zl'y-ZB which wherr closed; serve to connect by wayof an auxiliary relay. 29 a cur- .,1rent, source, suchasbattery 30, to energize the trip. windings" .l I; ,and, I 9.1" circuit. may, be Ltraeedfromthe battery '30; through. conductor 3 I,

5 5g contacts 3230f relay 2a,. conductor 33, coil 19;.con-

3 ductor 34, coil I! and conductor 35 to the battery 30.

It will be understood that the contacts 21, 28 of the differential relay 26 should be closed only in the event of an internal fault in the transformer l2. This condition is indicated by an unbalance in the voltages at the terminals of the current transformers and 2| with the result that sufficient current flows through the coil to close the contacts 21, 28. Such unbalance may occur however during the period when the transformer I2 is being energized as by the closure of circuit breaker l I. It is to prevent this improper tripping of the circuit breaker H that the present invention is particularly addressed.

Reference is now made to the curves shown in Figs. 2 and 3 of the drawings. In Fig. 2 a few cycles of normal current i are shown, it being assumed that the waves are sinusoidal in form. An electromagnet or coil energized by this current exerts a force proportionate to the square of this current as shown by the broken line curve It, the force It having a repetition rate double that of the system frequency. On the other hand, the current resulting from circuit closing of the transformer may be unsymmetrical, as shown by the curve to of Fig. 3. The square of this current is shown by a superimposed Wave, as indicated by the broken line curve R0. The force 760 of an electromagnet or coil energized under these conditions is therefore alternately large and small. The ratio is such that the small impulses can be disregarded for all practical purposes when compared with the larger impulses. pulses occur with double the system frequency under normal current conditions, they are of the system frequency during the period that the transfoi'i'ner i2 is being energized. Advantage is taken of this fact in the control arrangement shown in Figure l which will now be described.

The control system includes the differential relay already referred to and indicated generally by the reference numeral 26, this relay being rendered inoperative when the current waves of the power circuit depart from their normal substantially symmetrical form. For this purpose one contact arm of the relay 2B is a spring member having an. inherent frequency of vibration other than the current wave frequency, i. e. other than 120 cycles per second in the case of a 60 cycles per second alternating current distribution system. In the illustrated embodiment, the relatively stationary contact arm 36 is a flat spring member carrying a contact 21 for cooperation with a contact 28 on arm 31 which is pivoted on a shaft 38 and has an extension or equivalent armature means 39 actuated by the relay coil 25. The mechanical tuning of the spring arm 36 may be coarse, or the tuning may be clamped by an adjustable set screw 40 to respond to a frequency band including the normal power circuit frequency, 1. e., a frequency of one-half that of the positive and negative halves of the current wave of the power distribution system.

The contacts 21, 28 of the differential relay 26 are in series in the energizing circuit of the coil 4| of the relay 29; the energizing circuit including the current source 30, lead 42, contacts '21, 28 of relay 26, lead 43, coil 41 and the return lead 44. The switch or contacts 32 of the auxinary relay 29 are in series in the lead 3| of the tripping circuit controlled by the main differential relay 26. A suitable time delay mechanism 45 is incorporated in the relay 29 to delay the Therefore, while the power imrestoration of the tripping circuit to operative condition after it is opened, by relays 2B and 29, when the energizing circuit of the transformer I2 is closed.

In the event of a transformer fault, a current i of the normal power circuit frequency flows in the differential circuit and energizes the coil '25 of the differential relay 2B. The movable contact arm 3! of relay 26 is rocked clockwise by a strong force resulting from current waves of double the power circuit frequency, and contacts 21, 28 engage under a relatively heavy pressure, thereby affording a substantially continuous energization of relay 29 to close the switch 32 in the tripping circuit, so that the tripping coils ll, 19 are thereby energized to trip the circuit breakers H and 15. The protective system thus functions in the normal manner to isolate the power transformer [2 in the event of a fault in the transformer.

The tripping circuit is not energized, however, by transient differential currents resulting from the closing of the circuit breaker ll to energize the power transformer 12. The differential current in in such event is not symmetrical but consists for all practical purposes in current waves of the power circuit frequency. The force lc that tends to close the contacts of relay 26 is thus more intermittent and of the frequency of mechanical vibration of the resilient contact arm 36. Arm 3? and its contact 23 rebounds from the resilient arm 36 and contact 21 during intervals between the major pulses of the asymmetrical current in, and the resilient contact arm 36 is set in vibration at its natural freq ency. The relay 29 is therefore intermittently energized by short current pulses when the contacts 21, 28 of relay 26 are periodically closed, but because of the vibration of contact 21 at the system frequency these highly intermittent current pulses do not develop a sufficient force in the coil 4| of relay 29 to overcome the effect of the retarding mechanism 45 on the armature of this relay with the result that the armature on this relay does not pull completely in. The relay switch 32 in the tripping circuit therefore does not close, and the energization of the main relay 26 by the differential current due to a closing of the transformer circuit does not effect an energization of the tripping circuit.

Since certain further changes can be made in the foregoing constructions and different embodiments of the invention can be made without departing from the spirit and scope thereof, it is intended that all matter shown in the accompanying drawing and described hereinbefore shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A device for protecting alternating current power apparatus including inductively coupled windings subject to transient magnetizing currents such as transformers and the like, and which are connected and disconnected from the power system by a circuit breaker having an electro-responsive tripping device therefor, said protective device comprising, a trip relay controlling energization of the circuit containing the tripping device of the circuit breaker, said trip relay having a delay closing characteristic, and an energizing circuit for said trip relay controlled jointly by contact means of a differential relay energized in accordance with the respective current flows in the inductively coupled windings of the power apparatus and by contact means of an electro-nechanical vibration contactor resonant at the frequency of the power system and operated therefrom.

A protective device as defined in claim 1 wherein said mechanical vibration system is coarsely tuned to resonance over a narrow band of frequencies bracketing the frequency of the power source.

3. A protective device as defined in claim 2 and which further includes an adjustable damping device for determining the band of frequencies over which said vibration system is resonant.

4. A protective device as defined in claim 1 wherein said mechanical vibration system is constituted by a pair of contact members, one of which is carried by a spring arm resonant at the frequency of the power system.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,898,752 Wellings Feb. 21, 1933 1,927,794 Fallon Sept. 19, 1933 

